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Ballast Tubes and Resistance Line Cords

From about 1933 many American (and some European) radios
were designed to operate directly from line voltage. These
were called AC-DC or "transformer-less" sets. The voltage for
heating the tube filaments was supplied directly from the
line. Since the mains voltage in America is 117 AC and the
tube filaments required only about 6 or 12v (25, 35, or 50v
for many rectifier and output tubes) a resistor had to be
provided to drop the excess mains voltage to the proper value
to operate the filaments. The problem was simplified by
connecting the tube filaments in series so that each filament
acted as a resistor, dropping the voltage somewhat for the
remaining filaments. In most cases, however, the line voltage
was greater than the sum of the voltages required to heat the
tube filaments and so an additional resistor was required to
drop the excess voltage.

Example: A typical five tube set of the mid-1930's
used three 6.3v tubes and two 25v tubes connected in series.
The total voltage drop was (3 x 6.3) + (2 x 25) = 69v. Line
voltage is 117v, Therefore, (117 - 69) = 48v must be dropped.

The resistor dropping the voltage often had to dissipate
considerable power and so would become extremely hot. In many
cases the resistor was built into the line cord, in the form
of a resistance wire running the entire length of the cable
("resistance line cords"). Since the heat generated was
distributed along the entire length of the cord, the cord
became only moderately warm and so posed little danger. In
other cases the resistor was built into a tube called a
ballast tube. These were glass or metal tubes that looked like
ordinary radio tubes and plugged into a socket on the chassis.
They were often identified by a code, such as the following:

RMA BALLAST CODEExample: type B-K-55-B-GThe
first letter (B) indicates a ballast tube but may not appear.
The second letter (K) indicates the type of pilot light used
in the set:

K - a 6-8v 0.15A bulb is used.L - a 6-8v 0.25A bulb is
used.

In practice, any type of bulb can be used without harm to
the set.

The number (55) designates the voltage drop in the
resistor, including that for the pilot light.

The letter following the voltage drop (B) indicates
the circuit and base wiring (see below).

The last letter (G) indicates a glass tube and may be
disregarded.An "X" after the lamp-designating letter
indicates a four-prong base e.g. LX55H.

In the wiring diagram below the numbers stand for the prong
connections of an octal socket.

With glass four-prong base ballast tubes, a much-used
system was to have a number indicating the overall.
resistance.

Example: type 185R4. The unit has an overall
resistance of 185 ohms. To convert this to the RMA code
multiply the resistance by 0.3 to obtain the voltage drop: 185
x 0.3 = 55v. The tube is equivalent to type KX55B.

The section of resistor across which the pilot lights are
connected serve to provide an electrical path in case the
lamps burn out. Also, it helps to absorb the surge in current
that occurs after the set is turned on while the tubes are
warming.

In the early 1940's tubes were designed with filaments that
operated on higher voltages so that a set of these connected
in series could operate off the line voltage with no dropping
resistor being required.

Burned-out ballast tubes should not be replaced by
resistors mounted in a set because a great deal of heat (15-25
watts) is generated.

In many cases the ballast tube or resistance line cord can
be replaced with a silicon diode (e.g. 1N4004), with a zener
diode (e.g. SK6X0) across the dial lamps to prevent them from
burning out.

The power dissipated in a ballast tube or resistance line
cord can be found by multiplying the voltage drop across them
by the current drawn by the tube filaments.

Example: The five-tube set considered previously
required a voltage drop of 48v. The current drawn by the tube
filaments is 0.3A. The power dissipated is (48 x 0.3) = 14.4
watts.